The present invention relates to a process and apparatus for producing molded articles formed from thermosetting epoxy resin mixtures.
Apparatus for the processing of thermosettable epoxy resin mixtures used heretofore consisted of a heated mold, generally having two separable parts, a locking device for sealing the mold, a pressure hose and valve element, and a pressure vessel. Liquid mixtures of epoxy resins were premixed in an ordinary vacuum mixer and consisted of resin, hardener, mineral fillers, short fiberglass filaments having a length of 1 mm or above, and accelerator. The mixture was introduced at room temperature into the pressure vessel and was injected therefrom at a pressure of 2 atmospheres or above by means of the pressure hose into the mold which was generally preheated to 150.degree. C. or above. The resultant temperature rise of more than 140.degree. C. accelerates the reaction of the resin mixture so that curing progressed sufficiently in less than 35 minutes for the molded piece to be rigid enough to be removed from the mold.
A drawback of this process was that the compressed air which filled upper portions of the pressure vessel penetrated the upper layer of the liquid therein which, in the case of an affected surface area of 2000 sq.cm. and an average viscosity of the molding resin compound of 15,000 cP, resulted in air penetrating the liquid to a depth of approximately 20 mm. The consequent inclusion of bubbles in the molded articles obtained from this gas-saturated layer of molding compound could not be avoided. In electrical components, where the molded articles were to be employed as e.g. support insulators for high voltage applications, the ultimate electrical strength of such articles was impaired. Additionally the presence of voids in the molded articles also led to partial discharges or arcing when electrical fields were applied.
In order to achieve satisfactory crosslinking with short fiberglass filament-filled resin mixtures; the resin binders must be subjected, prior to final mixing to a heat setting with pure epoxy resin, which complicates the preparation process.